The brain has within it, fluid filled chambers called ventricles. A schematic illustration of the ventricles in the brain is illustrated in FIGS. 1 and 2, which are an anterior and left lateral view of the brain respectively.
The fluid which surrounds and cushions the brain and spinal cord is called cerebrospinal fluid or CSF. The ventricles are interconnected and are critical in maintaining the structure of the brain and altering pressure within the brain which is known as the intra-cerebral pressure (ICP). In the event of a change in intra-cerebral pressure (generally following some type of injury to the brain which could be caused by a knock or infection for example) it may be critical to monitor this change and if necessary, draw off some of the cerebrospinal fluid located in the ventricles in order to decrease the intra-cerebral pressure.
Intra-ventricular catheters have been used for more than 20 years to monitor cerebrospinal fluid pressure by connecting them to a pressure transducer. If the pressure is high it is possible to draw off fluid to decrease the intra-cerebral pressure.
The catheters used are typically soft but have a firm, metal stylet which runs through the centre of the catheter. The stylet is typically withdrawn once the catheter is in the correct position.
The process of placement of an intra-ventricular catheter is illustrated in FIGS. 3 to 6. As illustrated in FIG. 3, an incision 10 is typically made in the skin of the patient's head 11. A burr hole 12 is drilled into the bone of the patient's skull to create an opening for the catheter as illustrated in FIG. 4. The skin of the head is spread using retractors and a catheter is inserted manually through the burr hole 12 as illustrated in FIG. 5. The medical practitioner must manually insert the catheter 13 such that the lower end of the catheter 13 sits in a ventricle 14. A pressure transducer 15 is then connected to the outer end of the catheter to monitor the intracranial pressure as illustrated in FIG. 6.
One problem with catheters and their placement occurs if the angle of entry is not perpendicular in all planes, then there is the possibility of both a horizontal and vertical error from the desired destination point. This is illustrated schematically in FIG. 7 showing the insertion of a catheter at an angle through a patient's skull 16. Vector mathematics illustrate that a significant error rate exists in the correct placement of catheters manually.
The average length of insertion of intra-ventricular catheters placed into the intra-cerebral ventricles generally ranges from 5 to 8 cm (but is of course limited by the distance of the ventricle from the skull and the size of the ventricle). A greater angle of error at introduction, leads to a greater increase in both horizontal and vertical error as illustrated in FIG. 7. An increase in depth of penetration also exacerbates the horizontal and vertical error.
Expressed in a tabular form, Table 1 below, shows the increased horizontal and vertical error with angle of deviation from perpendicular and according to depth of penetration.
TABLE 1Horizontal and vertical error with angle of deviationfrom perpendicular and depth of penetration.Angle of DeviationDepth ofHorizontalVertical(degrees)Penetration (cm)Error (cm)Error (cm)1050.870.071061.040.091071.20.11081.390.122051.710.302062.050.362072.380.422082.740.483052.50.6630630.83073.50.9430841.1
Misplacement of an intra-ventricular catheter may lead to a failure of the catheter to function, or may inadvertently damage delicate structures within the brain causing neurological injury.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.